1. Field of the Invention
The present invention relates to a document reader.
2. Description of the Related Art
In related art document readers, shading correction is performed to a read image using shading correction data in order to reduce variation (“shading”) in the amount of light received along the longitudinal direction (i.e., the main scanning direction) of a linear sensor which reads a document. The shading correction data is generated in accordance with read luminance distribution in the main scanning direction in which a white component is read before the reading of the document is started. Usually, control is made to read the document after the generation of the shading correction data in response to a user instruction to start reading of the document. Recently, a method of starting generation of the shading correction data including switching-on of a light source in response to a user manipulation with which an instruction to start reading of the document other than an instruction to start reading of the document may be recognized in order to shorten the time to elapse before reading of the document is completed has been proposed (see the U.S. Patent Application Publication No. 2008/0278774). The user manipulation with which an instruction to start reading of the document may be recognized includes placing a document on a document table, an input operation other than the instruction to start reading of the document to a manipulation unit and closing a pressure board.
Recently, more compact automatic document feeders (hereinafter, “ADFs”) and increased document reading speed contribute shorter time to elapse before a document fed from a document tray of the ADF reaches a document reading point. Therefore, in an apparatus described in the U.S. Patent Application Publication No. 2008/0278774, generation of shading correction data is started in response to a user manipulation with which an instruction to start reading of the document may be recognized in order to shorten the time to elapse between an issue of a user instruction to start reading of the document and completion of reading of the document.
It is not always enough, however, to start generation of shading correction data in advance of an issue of the instruction to start reading of the document. Since reading ability of a linear sensor easily varies due to self temperature rise at the start of the driving of the linear sensor, it is not desirable to start generation of the shading correction data in advance of the instruction to start reading of the document in a case in which generation of the shading correction data is completed between an issue of the instruction to start reading of the document and reading of the document is started. This is because, in this case, image quality may be impaired due to variation in reading ability after the generation of the shading correction data without any shortening of the time between an issue of the instruction to start reading of the document and the completion of reading of the document.